Dual Element Hot Water Cylinder

Alaroy · December 2023

Hi All,

I'm on-line from down in New Zealand. We used 240vAC power here but there's good interest in solar for water heating. Most of the systems use water filled, black painted copper panels and pump water through to the hot water cylinder. It's heavy and risks roof issues if there is a leak.

I've been thinking for a number of years if there's scope for a combination 240v AC / 24V DC hot water cylinder, running two heater element.

The primary element would be the 240v, connected to the house supply and with a thermostat, switching power off when it got too hot. The second would be the 24v element fed directly from solar panels on the roof, and generating permanently.

I'm not an electrician but I am an inventor, normally designing and building some pretty cool airport ground handling gear, however power security worries me. Much of our country's power is hydro, with a bit of thermal (steam) and one coal fired power station, for which we import dirty burning coal from offshore, because some of the political parties here won't let us mine our own. Government's pushing for more EV's and we just don't seem to have the line or generating capacity, so I'd like to do something positive about it.

If the sun is generating power though the PV panels and it's going into the hot water cylinder (ours is 360 litres for example), I figure the cylinder would be acting as a Solar Battery, storing up that power in the form of hot water (and in NZ hot water heating is approx 60% of the household power bill). I'm thinking that the PV system would work all day and at least keep the cylinder temp up, while the switched 240v system would come in via the thermostat and generate mains powered heat if the temp dropped. My belief is, even if the PV system doesn't get the water super hot, it would still help keep it warmer - any heat it generates surely means less has to be made using mains electricity?

I've seen 24v heater elements in the USA on line, but not here. I wondering if anyone thinks what I am suggesting might work, and is it feasible to run two separate heater systems through the same cylinder? It might be wishful thinking, but if it could work, surely it's got to reduce mains power use? For all I know you may well be able to by those sort of systems off the shelf in the USA, but I just haven't seen one here.

It seems to me, that give the high cost of power walls, this is potentially a cheaper and greener option that might just make a difference.

I'd welcome your comments please.,

Kind regards,

Alan

New Zealand

BB. · December 2023

Welcome to the forum Alan.

Using Hot Water Tanks as your energy storage vs batteries--That is not uncommon over here. There are some solar power charge controllers that have a mode that will turn on an aux load signal that allows the unneeded energy for battery charging from the PV Array to be used for other loads such as water pumping and/or water heating. Not a bad way to go if 60% of your raw energy needs are just for water heating.

A bigger issue can be that DC current switching is much more difficult vs AC power switching--Basically DC current makes for a much better arc welding effect vs AC power--If using mechanical relays, need heavier duty DC rated units if doing DC power management.

Also, you did not say what the wattage of your heating elements need to be... You can get larger "Cal Rod" type elements in the 4,800 Watt (or larger?) range for 240 AC, just run them on 24 VDC with a fraction of the Wattage. Just a quick sanity check:

Using a 2,000 Watt @ 120 VAC water heating element--Largest I could find at my local USA Home Depot (should be equivalent resistance to an 8,000 Watt @ 240 VAC element--If you have larger wattage 240 elements down south)

P=V^2 / R
R (heating element) = V^2 / P = 120V^2 / 2,000W =7.2 Ohms resistance for 2,000 Watt @ 120 VAC element
P=V^2 / R = 24V^2 / 7.2 Ohms = 80 Watts

A quick search in New Zealand, I found this 2x3,000 Watt unit (6,000 Watts total) @ 240 VAC (I guess--No voltage specs listed):

https://smithsnz.co.nz/collections/water-heating/products/swe66-incoloy-6kw-c-w-short-thermister-dble-blade-spa-260mm

That is one option (running high Wattage/High Voltage elements at lower voltages)...

However, as solar power systems increase in size, there is almost no reason to run at low voltage DC... You can run higher voltage PV Arrays with MPPT (maximum power point tracking) type Solar Battery Chargers that have models that will accept anything from 100 Volts Vmp array to even upwards of 400 Volts Vmp arrays (different models, voltage suggestions are approximate). Now you can run your 240 VAC element directly from a solar array (at least for now, solar panels are at historically cheap prices--So you can series/parallel a 'high voltage" array) and run the same array to power both your MPPT solar charger and a DC direct water heating element at high wattages.

Otherwise, DC to AC inverters are getting pretty cheap these days... Just run a normal Array>MPPT-Batt-Charger>Batttery-Bank>AC-Inverter and just switch on the 240 VAC AC water heater element(s) once the batteries are full. Basically a "dump" load (the water heater) for a solar power system (switch on/off the AC heating to keep the battery bank near full charge--The solar dump load). Now you are in the AC power switching zone and can use standard AC relays (mechanical or solid state) pretty easily.

Of course, if the tank is pressurized, you still need the TP Valve (temperature/pressure relieve valve) for safety. And usually a tempering valve (to prevent scalding hot water at the home water faucet by mixing cold and hot tank water to nominal):

https://www.wattsnz.co.nz/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/56892-source/tpr-valve-data-sheets
https://www.archerplumbing.co.nz/resources/all-you-need-to-know-about-tempering-valves

Note that there are still details to be worked out... For example, a single PV Array driving both a MPPT solar controller and a resistive water heating element may "confuse" the MPPT controller (I don't know for sure if will work well or not--System may work fine). And if you use DC current switching--That is the whole "DC switching can of worms" issue.

If you already have a good sized AC inverter--Just keeping with the 240 VAC standard heating element (sized for your needs--Low, medium, or high wattage depending on your home power system)--Is probably the easiest to start with.

And keep safe... Hot water and pressurized hot water systems can be very dangerous if redundant safety systems are not in-place.

Your thoughts Alan?

-Bill

Alaroy · December 2023

Hi Bill,

Merry Christmas for a start.

Thank you so much for taking the time to outline the options.. I need to get my head around this, but from what I understand what would make sense is keeping just the single 240v element and having an automatic switching system on to that, coupled to a dc/ac inverter? Then we can use the solar power once the storage battery is charged up, with the 240vAC coming on line earlier if the PV wasn't giving enough power as in on a dull day. Hope I got that right?

The standard HWC in NZ all have tempering valves and normally a top vent pipe instead of a pressure relief valve, so that if the cylinder boils it vents up to air and squirts up onto the roof (as in, if we are using the wetback on our log burner and it's got the water to boiling temperature).

We're not too bad at home as we have an Osburn 2100 log burner with wetback, so hot water in winter is never an issue, though our power bill jumps up in summer when we aren't using the log burner and wetback.

Where we live in Canterbury there' s no need to have air con for cooling, however there are a lot of people living in relative poverty for whom using solar to reduce their mains power bills would be very welcome. I'd .love to find a solution that is cost-effective for them..

Kind regards

Alan

BB. · December 2023

Alan, yes... Just use the water heater as a "dump load" once the battery is full. There are simple solar controllers that can be configured as a dump controller (i.e., battery reaches 14.75 volts, or whatever you decide is "full" and discharge down to 13.2 volts or so--The battery is not discharging until it falls below roughly 12.8 volts (this is a 12 volt lead acid battery bank--Other chemistries and/or bank voltages would need their own settings).

You might also look at the Wattage of the element you choose for the water heater--A lower wattage element (such as a 1,000 to 1,500 Watt) may be a lower draw on your smaller AC inverter/battery batter bank.

If you want to use the 240 VAC utility power as backup--Certainly can be done--Just a transfer switch and some sort of relay/simple controller logic.

In the USA, most of our water heaters have two elements... A lower element and an upper element. Usually, the lower element heats the "whole tank". The upper element turns on if the "cold water" level gets near the top of the water heater--Which then switches to the upper element to heat a smaller portion of the water at the top of the tank.

https://www.beltzhomeservice.com/how-does-a-water-heater-work/ (electric dual element USA water heater)

In a solar+grid power situation, you could (for example) connect the top element to the grid (for backup power), and to the lower element for solar power... That way, if you use all of the lower 2/3rds of the hot water, then the start on the last 1/3rd, the upper element will turn on and run from grid power to give you enough hot water for kitchen and enough hot water for a shower.

Lots of do it yourself options with water heating... Including just running a solar array directly to the water heater and no-batteries.

This saves you the costs and expenses of running a battery bank. Cost you a "second" solar array for water heater. And then the DC switching issues for the PV array to heating elements (this is not a trivial question). Solar PV panels are historically cheap right now. And batteries are expensive.

I have not seen much in the way of Solar Water heating on the forum lately. Using thermal solar panels and a storage tank has been a good DIY project for many. But it seems the maintenance of these systems is a plumbing issue (leaks, corrosion, etc.) and if you are in a cooler climate, needing anti-freeze or drain back system during sub freezing weather. In our are, the few solar thermal systems seem to be abandoned in a few years or decade or so.

A nice article on a larger DIY home heating project (in a winter snow region):

https://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

And I have not seen much dedicated solar array to electric water heater projects either for quite a while. Maybe some members out there have more information.

-Bill

SumPower · December 2023

With the current low pricing of solar panels and dependablilty of off-grid inverters, I believe that using the output of the AC from the inverter is the most reliable method of heating hot water. Just my opinion.

We use the solar to get close to "charge parameters met" on the battery bank and then turn on the water heater manually at the breaker panel. It is only turned on when we are assured that the solar coming in is capable of charging the battery bank and serviceing the water heater load at the same time. We have an 8Kw array.

We have a set time to turn the water heater off each day, based on the time of year or if it is cloudy. (I live in an area that gets close to 295 days a year of sun). After the hot water heater is turned off we have enough hot water for all are needs until the next morning.

I have watched different approaches to heating hot water off grid over the years. My take on it, is using DC current and controlling it runs into the inherent arcing problem with DC circuits. Then reliability becomes a some what constant issue.

SteveK · December 2023

I also use the AC side for water heating but do see the attraction of a DC water heater dump load for those long summer days where there is much excess production. I use a Northroads/Nyle HPWH add-on module to our existing 60 gallon electric water heater. We no longer use the elements at all freeing up the 30A circuit entirely for other uses.

If I'm looking to dump some production I just let my system switch on the dehumidifier. If there is consistent overproduction i.e. midsummer I'll add another house circuit to the mix for a few months.

Dual Element Hot Water Cylinder

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